A cold crucible induction furnace is used to melt and heat electrically conductive materials placed within the crucible by applying an alternating magnetic field to the materials. A common application of such furnace is the melting of a reactive metal or alloy, such as a titanium-based composition, in a controlled atmosphere or vacuum. FIG. 1(a) illustrates the principle features of a conventional cold crucible furnace. Referring to the figure, cold crucible 100 includes slotted wall 112. The interior of wall 112 is generally cylindrical. The upper portion of the wall may be somewhat conical to assist in the removal of skull as further described below. The wall is formed from a material that will not react with a hot metal charge in the crucible, when the crucible is fluid-cooled by conventional means. For a titanium-based charge, a fluid-cooled copper-based composition is suitable for wall 112. Slots 118 have a very small width (exaggerated for clarity in the figure), typically 0.005 to 0.125-inch, and may be closed with a heat resistant electrical insulating material, such as mica. Base 114 forms the bottom of the cold crucible. The base is typically formed from the same material as wall 112 and is also fluid-cooled by conventional means. The base is supported above bottom structural element 126 by support means 122 that may also be used as the feed and return for a cooling medium. A layer of heat resistant electrical insulation 124 (thickness exaggerated in the figure) may be used to separate the base from the sidewall. Induction coil 116 is wound around the exterior of wall 112 of the crucible, and is connected to a suitable ac power supply (not shown in the figure). When the supply is energized, current flows through coil 116 and an ac magnetic field is created within and external to the coil. The magnetic flux induces currents in wall 112, base 114 and the metal charge placed inside the cold crucible. Flux penetration into the interior of the crucible is assisted by slots 118. Heat generated by the induced currents in the charge melts the charge. As illustrated by furnace 100 in partial detail in FIG. 1(b), a portion of metal charge adjacent to the cooled wall and base freezes to form skull 190 around liquid metal 192. The skull acts as a partial container for the molten metal, and the upper regions of the molten metal are at least partially supported by the Lorentz forces generated by the interaction of the magnetic field produced by coil 116 and the induced currents in the metal charge, to form a region of reduced contact pressure or even separation 194 between the wall and the liquid metal. Such reduced contact pressure or separation is important in reducing the thermal losses from the hot charge to the cold crucible. The Lorentz forces also cause the liquid metal to be vigorously stirred. After removal of the liquid metal product from the crucible, the skull can be left in place for a subsequent melt, or removed from the crucible, as desired.
As mentioned above, liquid metal in the crucible above the skull is generally kept away from the crucible's wall by Lorentz forces acting on the mass of liquid metal. Fluid motions caused by induced currents can intermittently disturb the region of separation between the wall and the mass of liquid metal. Such disturbances increase the boundary area of the melt, resulting in increased heat radiation losses from the liquid, or even increased conduction losses, if some of the liquid metal washes or splashes against the wall of the crucible.
It is sometimes desirable to superheat the liquid metal, for example to make it more fluid and therefore, more suitable for casting into a mold to form a casting having thin sections. However, the above apparatus and method has disadvantages when used to superheat the liquid metal. With increased superheat, there is an increased temperature difference between the liquid metal (melt) and the skull. This results in an increase in the heat transferred from the liquid metal to the skull. Consequently a portion of the formed skull melts back to liquid metal, which reduces the thickness of the skull. Decreased skull thickness increases heat losses from the liquid melt. Further the skull may be reduced in overall volume, so that parts of the liquid melt formerly contained within the skull can come into contact with the wall of the crucible, which greatly increases the heat loss from the liquid metal. In practice, the result is that for any reasonable power input to the above apparatus and process, the superheat is severely limited.
V. Bojarevics and K. Pericleous, Modelling Induction Skull Melting Design Modifications, Journal of Materials Science: Special Section: Proceedings of the 2003 International Symposium on Liquid Metals, Vol. 39, no. 24 (December 2004), pp 7245-7251 (presented on 23 Sep. 2003 in Nancy, France), suggests locating a separate dc coil adjacent to the ac coil of a cold crucible arrangement (paragraph beginning at the bottom right-hand column on page 7248 and continuing on page 7249 page 4 of the Bojarevics and Pericleous paper); i.e. towards the bottom part of the crucible and below the ac coil. DC current flowing through the dc coil creates a dc magnetic field that is superimposed on the ac field. When the molten charge, driven by the Lorentz forces previously described, moves across the field lines of the dc field, additional currents are induced in the moving metal. Such currents react with the dc flux to produce a braking action that reduces the fluid velocity. Such braking action is well known and is often referred to as eddy current braking or eddy current damping. By reducing the metal flow velocity, such damping reduces the turbulence in the liquid metal near the bottom of the cold crucible, thereby reducing the heat convectively transferred from the liquid metal into the skull; thereby permitting significantly increased superheat for a given power input. Such use of a dc magnetic field for eddy current damping or braking of moving metal in an induction coil is known prior art (see e.g. U.S. Pat. No. 5,003,551). However, locating a dc coil adjacent to the ac coil as proposed in the Bojarevics and Pericleous paper, would result in the ac magnetic field inducing high losses in the large cross sectional dc conductors shown in the paper. Moreover, there is no recognition or analysis of this deleterious effect in the Bojarevics and Pericleous paper. Nor can this problem be alleviated by simply moving the dc coil away from the ac coil, or vice versa, because the magnetic field of a coil so moved would be reduced in the crucible's interior space, thus rendering the moved coil less effective.
Therefore, there exists the need for apparatus and a method of induction melting an electrically conductive material with a cold crucible wherein convective heat loss to the cold crucible is limited, in order to obtain more superheat.